1. Field of the Invention
The present invention relates to microlithography employing the electron projection lithography (EPL) technique that is used in particular to produce masks to be used in the fabrication of microelectronic components with submicron dimensions. The invention is intended to be used in particular to etch 65 nm to 45 nm patterns in semiconductor materials. The invention relates more particularly to a device used to fabricate a mask by plasma etching a semiconductor substrate, and in particular to monitoring the uniformity of the depth of etching and the uniformity of the corners of the patterns over the whole of the surface of the substrate.
2. Description of the Prior Art
Silicon substrates are currently micromachined using plasma etching techniques such as that described in the U.S. Pat. No. 5,501,893, which consists in partially protecting the silicon substrate by an etching mask and submitting the substrate partially protected in this way to an alternating succession of plasma etching steps using an etching gas and plasma passivating steps using a passivating gas. To fabricate the etching mask, a pattern to be etched is transferred onto the substrate from a transmission mask. The transfer is usually effected by the photolithographic technique of exposing the substrate to photonic radiation through the transmission mask. The transmission masks themselves are fabricated from d master mask that is usually produced by means of a very fine beam of electrons, with beam dimensions less than the smallest dimensions of the pattern to be obtained, controlled by computer means, which sweeps the surface of an electrosensitive resin substrate to draw the required pattern thereon. This kind of operation takes a very long time. The transmission masks are therefore produced by a different technique, for cost reasons in particular.
Semiconductor component miniaturization and complexity requirements have increased in recent years. Photolithography as used at present has reached its limit in terms of line resolution. Lithographic techniques are coming into use that employ shorter wavelengths, such as electron or ion beams and X-rays.
The masks employed when exposing semiconductor substrates to an electron beam generally take the form of an array in which small areas carrying the pattern to be etched are separated by an array of thicker so-called grillage areas. These small areas are on a thin membrane of the order of 2 μm thick. Examples of such masks are described in U.S. Pat. No. 6,352,802, for example. A beam of electrons having a section of the same order of magnitude as the small area projects onto the mask a reduced image of the patterns to be etched through the small areas onto the substrate intended to become a microelectronic component. To obtain a correct projected image of the mask, it is essential that the membrane constituting each of the small areas is of constant thickness.
It has been found that, when plasma etching transmission masks, the membrane is thicker at the edges of the mask than at the center. This defect leads to non-uniform exposure of the semiconductor substrate through the mask, which leads to unequal etching of the substrate and therefore the rejection of these products.
An object of the present invention is to propose a device and a method for obtaining a constant membrane thickness over the entire area of the transmission mask for use in electron beam microlithography during the fabrication thereof.